This invention relates to aluminous abrasive grits and particularly to sol-gel alumina abrasive materials with improved grinding performance.
Sol-gel alumina abrasives are conventionally produced by drying a sol or gel of an alpha alumina precursor which is usually but not essentially, boehmite; breaking up the dried gel into particles of the desired size; then firing the pieces to a temperature sufficiently high to convert them to the alpha alumina form. Simple sol-gel processes are described for example in U.S. Pat. No. 4,314,827; 4,518,397; 4,881,951 and British Patent Application 2,099,012.
In a particularly desirable form of sol-gel process, the alpha alumina precursor is "seeded" with a material having the same crystal structure as, and lattice parameters as close as possible to, those of alpha alumina itself. The "seed" is added in as finely divided form as possible and is dispersed uniformly throughout the sol or gel. It can be added ab initio or it can be formed in situ. The function of the seed is to cause the transformation to the alpha form to occur uniformly throughout the precursor at a much lower temperature than is needed in the absence of the seed. This process produces a crystalline structure in which the individual crystals of alpha alumina, (that is those areas of substantially the same crystallographic orientation separated from adjacent crystals by high angle grain boundaries), are very uniform in size and are essentially all sub-micron in diameter. Suitable seeds include alpha alumina itself but also other compounds such as alpha ferric oxide, chromium suboxide, nickel titanate and a plurality of other compounds that have lattice parameters sufficiently similar to those of alpha alumina to be effective to cause the generation of alpha alumina from a precursor at a temperature below that at which the conversion normally occurs in the absence of such seed. Examples of such seeded sol-gel processes are described in U.S. Pat. No. 4,623,364; 4,744,802; 4,954,462; 4,964,883; 5,192,339; 5,215,551; 5,219,806 and many others.
Many of the patents in this area focus on the use of additives, (which are generally refractory oxides), to modify the properties of the grain, particularly the hardness. Many of these appear to act as crystal growth modifiers or as generators, in conjunction with the alumina, of spinels or magnetoplumbite structures within the alumina crystals or at the grain boundaries. About the only metals oxides that have been excluded have been the alkali metals, which have, since the earliest patent on the subject, been considered detrimental to the performance of sol-gel alumina abrasives. The one exception to this has been U.S. Pat. No. 5,190,567 which teaches that lithium oxide is surprisingly found to be effective to produce fine microstructures.